1. Field of the Invention
This invention relates generally to output drivers and, more specifically, to an output driver that minimizes source-point reflections when driving a transmission line by generating a constant source impedance.
2. Description of the Prior Art
When a driver sends a signal down a transmission line, the signal will travel down the transmission line unaltered until the signal reaches some discontinuity. The discontinuity may be caused by the end of the transmission line, a change in the width of the transmission line, or for any of a number of other reasons. The discontinuity will cause echoes or reflections to be generated which may be sent back to the driver which in turns reflects the echoed signal back on the transmission line to a load.
In order to reduce the reflections on the transmission line, transmission line termination was used to simulate a continuous unaltered transmission line. Termination of the transmission line may occur on either end of the transmission line. For example, a resistor may be placed on the end of the transmission line furthest from the driver. When the driver sends the signal wave on the transmission line, the signal wave will travel down the transmission line until the signal wave hits the resistor. The energy of the signal wave is absorbed by the resistor and no reflection is generated.
By terminating the transmission line at the end furthest from the driver, several problems are created. If a point load is placed in the middle of the transmission line, a partial reflection will be generated by the point load. The partial reflection will be sent back to the driver and can be reflected back around from the driver to the point load. Furthermore, the use of a shunt resistor at the end of the transmission line complicates the integrated circuit (IC) board layout. Shunt resistors consume valuable silicon real estate and require large amounts of power.
The simple solution to the above problem is to have controlled impedance at the source of the transmission line. However, in the implementation of a typical CMOS driver, the CMOS driver's output impedance is high if the CMOS driver is operating in a saturation mode. Since the CMOS driver is in a saturation mode in the incident wave state, reflections such as those from the point loads are reflected back in phase to the point loads. This increases the net settling time of the transmission line.
Therefore, a need existed to provide an improved output driver. The improved output driver must generate a constant source impedance. This will allow the improved output driver to minimize source point reflections when driving a signal on a transmission line.